1. Field of the Invention
This invention relates to solar cells, and more specifically to tandem solar cells that efficiently convert solar radiation into electrical energy.
2. Description of the Related Art
Solar cells are photovoltaic devices that use semiconductors to convert photons into electrical energy. In a semicondutor, a conduction band and a valence band are separated by an energy gap E.sub.g that varies with material composition and temperature. When a photon is absorbed by a semiconductor, an electron is promoted from the valence band into the conduction band, thereby creating a hole in the valence band. Since a hole represents the absence of an electron, it can be regarded as a positive charge carrier. When donor or n-type impurities (which increase the number of electrons available to carry current) are added to one side of a semiconductor crystal and acceptor or p-type impurities (which increase the number of holes) to the other, a p-n junction is formed that permits current flow in one direction but restrains it in the opposite direction. Thus, p-n junctions are ideal for converting solar radiation into electricity.
A photon of wavelength .lambda. (as measured in a vacuum) and frequency .nu. has an energy h.nu.=hc/.lambda. and is generally absorbed by a semiconductor when h.nu..gtoreq.E.sub.g. However, any extra energy in the photon (h.nu.-E.sub.g) is converted into thermal rather than electrical energy, since only one electron-hole pair can be created for each absorption event. On the other hand, a semiconductor is more transparent to wavelengths corresponding to energies less than E.sub.g, since in this case the photons are not energetic enough to promote electrons from the valence band into the conduction band. Thus, no single semiconducting material can convert the entire solar spectrum into electrical energy, since the most energetic photons produce largely thermal energy and are therefore inefficiently utilized, while the least energetic photons cannot be absorbed.
By cascading the p-n junctions of different materials, however, the overall conversion efficiency can be improved.
In so-called "tandem" solar cells, a top cell having a p-n junction of a high energy band gap semiconductor intercepts the most energetic photons. Lower energy photons pass through the top cell before they enter another cell having a p-n junction of a lower energy band gap semiconductor. In this way, an additional portion of the solar spectrum is used. In tandem solar cells, tunnel junctions are often used to connect p-n junctions, so that current can pass from one cell to the next. However, in a monolithic structure, the various layers must have thermal properties and lattice spacings which are compatible with each other. Otherwise, internal strains will be introduced during the epitaxial growth process, resulting in defects which will then propagate throughout the entire structure.
An example of a tandem solar cell is the GaInP/GaAs dual junction cell grown on a GaAs substrate, which is described by S. Kurtz et al. in "19.6% Electron-Irradiated GaInP/GaAs Cells," Proceedings of 1st WCPEC (World Conference on Photovoltaic Energy Conversion), pp. 2108-2111, 1994. This tandem solar cell produced 19.6% conversion efficiency of the air mass zero (AM0) solar spectrum, which is the pure solar spectrum with no atmospheric absorption. GaInP/GaAs cells have also been grown on Ge substrates, as reported by P. K. Chiang in "Large Area GaInP.sub.2 /GaAs/Ge Multijunction Solar Cells for Space Applications," Proceedings of 1st WCPEC, pp. 2120-2123, 1994. Chiang et al. predict the conversion efficiency to be slightly better when the Ge is active (i.e., the Ge and GaAs layers form a third junction), than in the dual junction case in which the Ge is inactive and acts simply as a mechanical platform (26.5 vs. 24.2% for AM0 at 28.degree. C.). This is so because the additional active layer results in more efficient use of the solar spectrum.
GaInAsP and GaInAs individually are better converters of the IR than Ge, as indicated by M. W. Wanlass et al. in "Development of High-Performance GaInAsP Solar Cells for Tandem Solar Cell Applications," Proceedings of the 21st IEEE Photovoltaic Specialists Conference, pp. 172-178, 1990. However, GaInAsP and GaInAs have lattice spacings quite different from Ge, so that it is not possible to construct a monolithic structure by simply epitaxially depositing one layer on top of another. Semiconductors having different lattice structures can be connected, however, by using transparent adhesive layers to connect semiconductor layers both mechanically and optically. (See, for example, J. C. C. Fan et al., "Optimal Design of High-Efficiency Tandem Cells," Proceedings of the 16th IEEE Photovoltaic Specialists Conference, pp. 692-704, 1982.) In this case, the cells must be connected with metallic parts such as wires or a metallic grid, rather than with tunnel junctions. However, there are intrinsic absorption losses associated with this approach. First, there are always some losses in any "transparent" adhesive layer, but more importantly, there are losses in semiconductors due to excess holes and electrons known as free carrier absorption.
A photovoltaic device which optically joins different semiconductor layers without using a transparent adhesive layer is described in U.S. Pat. No. 4,328,389 to Stern et al. In this approach, a first broadband reflector concentrates sunlight on a high energy band gap photovoltaic array that converts high energy photons into electricity. A second broadband reflector is positioned behind the first array and reflects and concentrates light that is not absorbed in the first array back through it towards a second photovoltaic array which has a low energy band gap and converts low energy photons into electricity. In this approach, however, low energy photons must pass through the first array twice before they reach the second array, resulting in absorption losses which can be significant if the density of free carriers in the first array is high. Thus, there is still a need for a tandem solar cell that reduces losses due to free carrier absorption. Further, the device of Stern et al. is only designed for use with solar concentrator systems and specifically avoids spectrally selective reflectors.